Citric acid is a hygroscopic organic acid used as a food acidulant, and in pharmaceutical, industrial and detergent formulations. The increased popularity of liquid detergents formulated with citric acid has been primarily responsible for the growth of this commercially valuable organic acid.
Citric acid is commercially produced by a culture fermentation process which employs molasses or a sugar such as glucose, sucrose or converted lactose as a substrate and a fungus such as Aspergillus niger or yeast such as Candida lipolytica as biocatalyst. The fermentation product typically contains biomass from the spent microorganism, carbohydrates, amino acids, proteins and salts as well as citric acid which must be separated from the fermentation broth to provide a pure product.
Citric acid salts, particularly the sodium salt, are suitable for use as chelators, flavor enhancers and buffers in pharmaceutical, food and industrial applications where a higher pH than citric acid is required. These salts are typically prepared by neutralizing a citric acid solution with a base containing the appropriate cation, e.g. NaOH.
The literature describes many techniques for the purification of the impure fermentation broth. Among the literature references is published European Patent Application 167,957 owned by Hoechst AG which discloses a process for isolating anhydrous acidic compounds by bringing a solution of the acid into contact with a weakly basic, adsorbant, ion exchange resin, preferably those containing tertiary amino groups, and then desorbing the acid with water and/or steam.
In Offenlegungsschrift DE 3502924, owned by Benckiser GmbH, a citric acid purification process involving membrane filtration, preferably ultrafiltration, together with adsorption of impurities on a non-ionic resin such as polystyrene or polyacrylamide and crystallization is described.
In U.S. Pat. No. 4,851,573 there is disclosed a method for separation of citric acid from its fermentation broth by contacting the broth with a water-insoluble macroreticular gel of a weakly basic anionic exchange resin possessing tertiary amine functional groups or pyridine functional groups and a cross-linked acrylic or styrene resin matrix. The citric acid is desorbed by water or dilute sulfuric acid.
European Patentschrift 163,836 discloses a process for the continuous preparation of granules having a narrow particle size distribution which involves spraying a slurry/solution of the material to be granulated into a fluidized bed by means of a spraying nozzle to form granules. Fine materials escaping from the fluidized bed with the off-gas are separated and returned to the fluidized bed as nuclei for further granule formation and the granules of the desired size are formed by adjusting the sifting gas stream. The finished granules are discharged via one or more countercurrent gravity sifters installed in the inflow plate of the fluidized bed apparatus. Among the materials which are disclosed as capable of being granulated in this manner is citric acid (Zitronensaure).
U.S. Pat. No. 4,275,234 to Baniel et al describes a method for the recovery of organic acids from their aqueous solution by an extraction process comprising a first extraction stage at which the aqueous solution is contacted with a water-immiscible extractant comprising a water-immiscible organic solvent and, dissolved therein, at least one secondary or tertiary amine in which the aggregate number of carbon atoms is at least 20 together with a back extraction step in which the organic extract, separated from the original aqueous solution, is stripped with an aqueous liquid at a temperature which is higher by at least 20.degree. C. than the temperature of the first extraction step.
Other methods used to purify citric acid fermentation broths include ion exchange, nanofiltration and the so-called lime/sulfuric method, i.e. where citric acid is recovered by the addition of a suitable source of calcium ion, e.g. calcium hydroxide (lime), to form the slightly soluble calcium salts of citric acid. This precipitate is further purified by treating it with sulfuric acid to yield calcium sulfate (gypsum) and a solution of free acid.
Regardless of the preliminary purification steps used in the manufacture of citric acid, the final step in its production typically includes a series of crystallization steps involving evaporation, crystallization and centrifugation equipment. A conventional crystallization scheme consists of a batch vacuum-pan evaporator or a forced circulating evaporator coupled with auxiliary tankage and appropriate centrifuge equipment. Within these systems the crystals formed are separated from the mother liquor and advanced to the next crystallization step which can be carried out in a batchwise or continuous operation.